The present invention relates to highly elastic polyurethane contact adhesives and a process to coat substrates with them. This invention requires a) a polyisocyanate with a functionality of less than about 4 and b) a polyol blend comprising 1) about 90 to 100% polyethers with molecular weights from about 1,800 to 12,000 and average functionality from about 1.5 to about 4, and 2) up to about 10% of chain extenders with molecular weights from about 60 to 400 and average functionality from about 1.5 to about 3. The functionalities and amounts of components a) and b) are such that the molecular weight between crosslinks is from about 7,000 to about 16,000 and the equivalent ratio of isocyanate groups to isocyanate-reactive groups is from about 80:100 to about 120:100 and the urethane group content is from about 1 to about 8%.
Polyurethane adhesives and sealants of various types are well known. See, for example, U.S. Pat. Nos. 3,779,794, 4,156,064, 4,624,996, and 4,625,012.
In certain applications, it is desirable to provide an elastomeric material that can be applied to a flexible substrate in the liquid state and cured to provide an adhesive layer. This allows this flexible substrate to be attached to and in some cases subsequently removed from another surface.
Materials of this type are described in, for example, U.S. Pat. Nos. 5,102,714 and 5,227,409. These patents relate to adhesives for adhering carpet onto flooring based on a) a polyol mixture having an average functionality of 2.2 or less and equivalent weight of at least 500 and containing from 10 to 70 mole percent of monoalcohol and optionally up to 10% of a chain extender with an equivalent weight of from about 30 to 500, with b) a polyisocyanate mixture having an average functionality of about 2.2 or less.
Adhesives of this type are not useful in certain applications because of their undesirable tendency to soften and lose adhesive properties at temperatures slightly above room temperature. This is especially undesirable when the flexible substrate will be attached to a vertical surface and can result in the substrate releasing from the surface when it is heated.
The polyurethane contact adhesives of the present invention have several advantages. They can be applied to the substrates in liquid form and cured to form permanently tacky elastomeric layers useful as contact adhesives. The softening points of the reacted adhesives is sufficiently high so that the compositions do not lose their adhesive strength and the substrates thus remain intact on vertical surfaces when heated above normal room temperatures. The adhesives have sufficient flexibility so that they are also useful on flexible substrates. Another advantage of the present invention is that the contact adhesives can be attached to and removed from a substrate, and are reusable.